1. Field of the Invention
The present invention relates to liquid crystal devices, and has particular but not exclusive relevance to spatial light modulators.
2. Discussion of Prior Art
It is known to incorporate anisotropic dichroic materials, or dyes, in a liquid crystal host material, and/or to provide liquid crystal material in which at least one liquid crystal component has a significant dichroism. In one type of device, known as a “guest-host” device, guest dye molecules align co-operatively with the molecules of a host liquid crystal material. Alteration of the liquid crystal alignment, for example by application of an electric field, causes the dye molecules to re-orientate, thereby altering the optical properties of the device, and in particular the absorption or colour.
However, it is also possible to manipulate the molecules, for example by exposing the dye molecules to linearly polarised light within the absorption band of the dye, so as to affect the liquid crystal alignment. The polarised light tends to produce an effective re-orientation of the dye molecules, and this in turn produces a small torque on the liquid crystal host material, pushing its director away from the polarisation direction of the incident light. Where the liquid crystal molecules absorb the polarised light directly, a similar effect can occur, but without the intermediation of the dye.
It is also known that the alignment of a liquid crystal phase adjacent the surface of a substrate is dependent inter alia on the properties of the substrate surface. Although a substrate surface is often treated, for example by rubbing a polymer film or by vapour deposition, so as to induce a single energetically preferred type of alignment at the surface, it is known that it is possible to treat a substrate surface so that there is more than one energetically favourable alignment thereat. The favoured alignments will be separated by intervening alignments which are less favoured energetically, i.e. there is an energy barrier between the favoured alignments. The favoured alignments may or may not be energetically equal, or equally favourable relative to the intervening alignments, and there is a degree of control of the height of the energy barrier as will be exemplified later. In certain cases, particularly where a first alignment is more energetically stable than a second alignment, the energy barrier may be sufficiently low relative to the second alignment that relaxation to the first alignment may occur in response to ambient or supplied thermal energy, for example, so that the second alignment is considered to be metastable.
In most cases at least, the surface will receive treatments corresponding to each stable alignment. The treatments may use the same type of treatment for each alignment, for example two differently aligned gratings or oblique vapour deposition from two different angles, or different types of treatment may be used for different ones of the alignments, for example a surface profile such as a grating for a planar alignment and a coating of material to induce homeotropic alignment.
Thus, one way of providing at least two favoured alignments on a substrate surface is to provide a suitable surface relief pattern, such as one defined by two sets of parallel lines (gratings) extending in different directions over a common area. Depending on depth, the resulting pattern may vary from an array of isolated pillars on a flat surface to a smoothly varying “eggbox” pattern. A typical spacing for each set of parallel lines is around one micron, and for two favoured alignments the two sets of parallel lines could intersect at 90° or a lower angle. The two azimuthal directions of the preferred liquid crystal phase alignments thereat would be expected to differ by a corresponding angle. This type of arrangement is known as “azimuthally bistable” and further details thereof will be found in our UK Patent No. 0744041.
In an alternative arrangement, the substrate comprises a surface relief pattern such as a one dimensional sinusoidal grating, which is designed to give stability to a planar alignment with a predetermined direction. Typically the surface corrugations of the grating are 1 micron or less in height and are formed using photolithography or embossing. Provided over the grating is a layer for inducing homeotropic alignment of the adjacent liquid crystal material.
The homeotropic alignment layer and the surface relief pattern compete in defining the bulk alignment of the adjacent liquid crystal. The alignment tends to be homeotropic or planar depending on whether the depth of the grating is respectively far less or far greater than its pitch, but by tailoring the depth to pitch ratio to lie between these two extremes a bistable region is entered where either alignment has a degree of stability (corresponding to respective energy minima as the alignment direction is altered, as indicated schematically in FIG. 1 as a plot of energy against liquid crystal tilt angle (shown by way of example only with homeotropic alignment energy minima lying either side of a planar alignment energy minimum). The two alignment directions may have the same azimuthal direction (i.e. lie in the same azimuthal plane), or they may not. Generally, the energy minima for the two types of alignment may or may not be equal, and in the latter case, either alignment may have the lower energy minimum corresponding to the more preferred alignment—this will, in part, be determined by the depth to pitch ratio of the surface relief pattern.
While the exact profile of the surface relief pattern appears to be relatively unimportant in achieving bistability in tilt angle, it may contribute to the energy barrier between the two preferred alignments and/or to the associated energy minimum for the planar alignment. It should be noted that FIG. 1 is an example of the more general case where two stable orientations, not necessarily planar and homeotropic, have different values of “tilt” or “zenithal angle”, and that the invention is not limited to the situation shown in FIG. 1, but extends to this more general case.
In this arrangement, the “planar” alignment may sometimes involve a relatively high tilt angle, and is sometime referred to as the defect state, because it is also characterised by a pair of line defects in the liquid crystal or nematic director. Arrangements having stable planar and homeotropic states in which the liquid crystal director lies in the same azimuthal plane are known as “zenithally bistable arrangements”, and further details thereof will be found for example in our UK Patent No. 2318422, and our published patent application PCT/GB98/03787 (WO 9934251).
It should be understood that other bistable surfaces will possess at least two preferred alignment directions which differ in both zenithal and azimuthal angle. For example, silicon oxide could be deposited obliquely from two different directions which also differ in both zenithal and azimuthal angle.
Although surface profiles in the form of gratings, formed for example by photolithographic techniques, have been specifically mentioned above, any other method of providing a surface profile could be used, such as by oblique evaporation.
Furthermore, it should be noted that although the two bistable arrangements specifically described above both involve a surface profile, the latter is not a necessary requirement for providing a plurality of stable alignments at a substrate. Any arrangement where more than one alignment is energetically favourable, whether imposed by use the same technique adapted for each preferred direction as in the example of crossed gratings above, or by the use of different techniques for each preferred direction as in the case of the grating and homeotropic surface treatment exemplified above, may be used.
In addition, it is not an absolute requirement that each part of the substrate surface is adapted to favour both alignments. For example, each two commingled sets of suitably dimensioned and/or shaped areas of a substrate may be treated to produce the different alignments, whereupon it may be arranged that either of the alignments, once adopted and favoured by one set of areas, will prevail over the alignment favoured by the other set of areas. Thus two interleaved sets of stripes may be treated differently to produce a favoured planar alignment for one set and either a favoured differently directed second planar alignment or a homeotropic alignment for the other set, using any of the techniques mentioned above, such as surface treatment, coating, and/or profiling.
By careful choice of the liquid crystal material, including the provision of appropriate additive(s) to the liquid crystal material, and as indicated above by suitably selecting and/or treating the substrate surface in known ways, it is possible to control the energy barrier(s) between the two preferred alignment states. Also, in an assembled device comprising a layer of liquid crystal material between the one bistable (or polystable) substrate and a further substrate, the alignment imposed at the surface of the other substrate may modify the energies of the stable states at the one substrate.